Certain integrated circuits, for example, operational amplifiers or reference voltage generators, are capable of drifting in temperature during their operation if some of their parameters are not very accurately adjusted. Even though the parameters of these circuits are calculated in order to avoid such drifts, technological manufacturing fluctuations result in that the various circuits manufactured on a same wafer or circuits manufactured on different wafers will exhibit technological variations such that the manufactured circuits drift in temperature.
An example characteristic of an integrated circuit capable of drifting in temperature if the parameters of this circuit are not accurately adjusted is the offset voltage between the inverting input and the non-inverting input of an operational amplifier.
Thus, to be able to provide integrated circuits which do not drift in temperature, manufacturers have to carry out wafer tests for each manufactured circuit and to adjust the parameters of the circuits so that they do not drift in temperature. Such an adjustment is for example performed by providing in the integrated circuits resistor networks where elements are disconnected by laser trimming.
More particularly, in the case of the offset voltage of an operational amplifier, four-point probe measurements are carried out at a plurality of temperatures, currently two temperatures, and the way to adjust the resistances of the above-mentioned resistance networks is determined according to these measurements. As an example, means for adjusting the offset voltage according to the measurements performed at various temperatures are described in the ANALOG DEVICES document entitled: MT-037 TUTORIAL, October 2008 (incorporated herein by reference).
This requires, as already mentioned, carrying out measurements at two different temperatures. To achieve this, the wafer is placed on a heating support. The support is first maintained at the ambient temperature and then heated to reach a high temperature, for example, 100° C. At each of these temperatures, four-point probe measurements are performed and the previously-described laser adjustment method can then be implemented.
A difficulty of such a method is to take the wafer to two different temperatures. In practice, passing by means of a heating support from a first stable temperature to a second stable temperature takes several minutes. This duration is far from being negligible as compared with the total chip manufacturing time and has a significant impact on the total manufacturing cost of a wafer.
The test time thus needs to be decreased.